System for selective homogeneous charge compression ignition

ABSTRACT

A method of operating an internal combustion engine may include selectively operating the engine in a first mode of operation, wherein fuel may be injected into compressed fluid to create a substantially heterogeneous mixture of fuel and compressed fluid, and fuel may be ignited by the heat of the compressed fluid. The method may also include selectively operating the engine in a second mode of operation, wherein fuel is injected into gas to create a substantially homogeneous mixture of gas and fuel, the substantially homogeneous mixture is compressed, and fuel is ignited by the heat of the compressed gas. The method may further include determining whether to operate the engine in the first or second mode according to engine conditions.

TECHNICAL FIELD

The present disclosure relates generally to internal combustion engines, and more particularly to the operation of a diesel engine in a homogeneous charge compression ignition mode.

BACKGROUND

A diesel engine may typically include a combustion chamber composed of a piston slidably mounted within a cylinder. The conventional combustion process in the diesel engine may be initiated by the direct injection of fuel into the cylinder after upward movement of the piston has created a volume of highly compressed air within the top portion of the cylinder. The fuel may be almost instantaneously ignited upon being injected into the highly compressed air, and thus may produce a diffusion flame or flame front extending along the plumes of the injected fuel. The production of the diffusion flame or flame front may result in the presence of pollutants in the diesel engine's exhaust stream. These pollutants may include, for example, particulate matter, nitrogen oxides (“NOx”), and sulfur compounds.

Due to heightened environmental concerns, engine exhaust emission standards have become increasingly stringent. The amount of pollutants in the exhaust stream emitted from the diesel engine may be regulated depending on the type, size, and/or class of engine. One method implemented by engine manufacturers to comply with the regulation of exhaust stream pollutants has been to employ a selective catalytic reduction (“SCR”) catalyst to clean NOx from the engine exhaust stream. Use of the SCR catalyst is disclosed in U.S. Pat. No. 6,823,660 to Minami. Minami also discloses that although the SCR catalyst has a high rate of clean-up (NOx reduction) at higher exhaust stream temperatures, the SCR catalyst's ability to clean drastically declines in idling or low engine load conditions, where the temperature of the exhaust stream falls. One reason for the decline is that the rate of NOx conversion is strongly affected by the temperature of the exhaust stream. Minami proposes raising the temperature of the exhaust stream before it passes through the SCR catalyst to provide near 100% removal of NOx. However, increasing the temperature of the exhaust stream in the manner proposed by Minami requires a rather complex arrangement of valve devices and conduits, and further, may require the use of extra fuel and/or energy.

On the other hand, allowing the exhaust stream to remain at lower temperatures may result in the presence of a greater amount of pollutants in the exhaust stream. As newer engines may also include a diesel particulate filter (“DPF”) to catch pollutants, allowing those pollutants to reach the DPF may cause clogging and may decrease the DPF's performance. Thus, the DPF may require frequent maintenance and/or regeneration to perform satisfactorily.

The system of the present disclosure is directed towards overcoming one or more of the constraints set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure may be directed to a method of operating an internal combustion engine. The method may include selectively operating the engine in a first mode of operation, wherein fuel may be injected into compressed fluid to create a substantially heterogeneous mixture of fuel and compressed fluid, and fuel may be ignited by the heat of the compressed fluid. The method may also include selectively operating the engine in a second mode of operation, wherein fuel may be injected into gas to create a substantially homogeneous mixture of gas and fuel, the substantially homogeneous mixture may be compressed, and fuel may be ignited by the heat of the compressed gas. The method may further include determining whether to operate the engine in the first or second mode according to engine conditions.

In another aspect, the present disclosure may be directed to an internal combustion engine. The internal combustion engine may include a combustion chamber including a piston and a cylinder, and a fuel injection controller that selectively operates the engine in first and second modes of operation, and selects the mode according to engine conditions. In the first mode, fuel may be injected into compressed fluid to create a substantially heterogeneous mixture of fuel and compressed fluid, and fuel may be ignited by the heat of the compressed fluid. In the second mode, fuel may be injected into gas to create a substantially homogeneous mixture of gas and fuel, the substantially homogeneous mixture may be compressed, and fuel may be ignited by the heat of the compressed gas.

In yet another aspect, the present disclosure may be directed to a work machine having an internal combustion engine. The internal combustion engine may include a combustion chamber including a piston and a cylinder, and a fuel injection controller that may selectively operate the engine in first and second modes of operation, and may select the mode according to engine conditions. In the first mode, fuel may be injected into compressed fluid to create a substantially heterogeneous mixture of fuel and compressed fluid, and fuel may be ignited by the heat of the compressed fluid. In the second mode, fuel may be injected into gas to create a substantially homogeneous mixture of gas and fuel, the substantially homogeneous mixture may be compressed, and fuel may be ignited by the heat of the compressed gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a diagrammatic view of a combustion chamber according to an exemplary disclosed embodiment.

FIG. 2 provides a closer view of the combustion chamber of FIG. 1.

FIG. 3 provides another view of the combustion chamber of FIG. 1.

FIG. 4 provides a flow diagram of a method, according to an exemplary disclosed embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to the drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 1 illustrates a combustion chamber 10 of an internal combustion engine 12. Although only a single combustion chamber 10 is shown, it should be understood that engine 12 may include multiple combustion chambers. Combustion chamber 10 may be formed by a cylinder sidewall 14, a cylinder end wall 16, and a reciprocating piston 18, and may include a combustion chamber longitudinal axis 20.

Reciprocating piston 18 may include a sliding plug 22 that may fit closely inside a bore defined by cylinder sidewall 14. Plug 22 may include a top surface 24 and a bottom surface 28 operatively connected to a crankshaft 30 by a connecting rod 32. Piston 18 may reciprocate within the bore by sliding back and forth between two positions while performing combustion cycles. The first position may be referred to as a top dead center position (“TDC”) corresponding to the position where plug 22 may be furthest from crankshaft 30. The second position may be referred to as a bottom dead center position (“BDC”) corresponding to the position where plug 22 may be closest to crankshaft 30. Accordingly, TDC and BDC may define the upper and lower extents of piston travel, respectively. Additionally or alternatively, the movement of piston 18 may be described according to the angular rotation of crankshaft 30 caused by movement of piston 18. For example, movement of piston from BDC to TDC may cause crankshaft 30 to rotate 180 degrees or one-half revolution. Thus, as piston 18 travels from BDC to TDC, it may be characterized as having traveled 180 degrees. The reciprocating movement of piston 18 between TDC and BDC may deliver power to crankshaft by ways known to those skilled in the art.

An intake port 34, intake valve 36, exhaust port 38, and exhaust valve 40 may be located about the cylinder end wall 16, as shown in FIG. 2. Intake valve 36 may be disposed in intake port 34 and may be operable to fluidly connect air intake port 34 with combustion chamber 10. Exhaust valve 40 may be fluidly coupled with combustion chamber 10 and may be operable to couple combustion chamber 10 with exhaust port 38. Intake port 34, intake valve 36, exhaust port 38, and exhaust valve 40 may function in ways known to those skilled in the art. Additionally or alternatively, intake port 34 and/or combustion chamber 10 may be configured to induce a swirl to fluids and/or gases in combustion chamber 10.

A fuel injector 42 may include a nozzle tip 44 extending directly into combustion chamber 10 through an opening 46 in cylinder end wall 16. As shown in FIG. 3, nozzle 44 may be configured to inject fuel such that fuel plumes 70 may be formed above a piston crater 26 in top surface 24 of plug 22. Fuel injector 42 may be concentric or parallel with the longitudinal axis 20 of combustion chamber 10, or may extend at an acute angle with respect to longitudinal axis 20 of combustion chamber 10. Further, fuel injector 42 may be of any conventional type. For example, fuel injector 42 may be of the mechanically actuated, hydraulically actuated, or common rail type, and may be designed for single mode or mixed mode operations. If intake port 34 and/or combustion chamber 10 is configured to induce a swirl, as discussed above, fuel injected by fuel injector 42 may be less likely to stick to cylinder sidewall 14.

In operation, fuel injector 42 may provide a combustible fuel to combustion chamber 10 as a function of a control signal. For example, during normal to high load engine operation, fuel injector 42 may receive a control signal causing fuel injector 42 to inject fuel into combustion chamber 10 one or more times. For example, fuel injector 42 may inject a first amount of fuel (a pilot injection) when piston 18 is located at any position between BDC and approximately 40 degrees before TDC. The pilot injection may mix with the intake air within combustion chamber 10, and the mixture may be compressed as piston 18 travels towards TDC position. The pilot injection/intake air mixture may combust when its combustion temperature is reached, such as, for example, when the heat of the compressed intake air is sufficient to ignite the pilot injection. Fuel injector 42 may also inject a second amount of fuel (a main injection) into the combustion chamber 10 contemporaneous with or slightly before the combustion temperature of the pilot injection/intake air mixture is reached in combustion chamber 10. The main injection may combust upon being introduced into the pilot injection/intake air mixture, due at least in part to its exposure to the heat associated with the pilot injection/intake air mixture. This may be referred to as a first mode of operation for engine 12. Additionally or alternatively, in the first mode, the pilot injection may be omitted. In such an embodiment, the intake air may be compressed by piston 18 as it travels towards TDC position. The main injection may be injected into the compressed intake air. If the compressed intake air has reached the combustion temperature, then the main injection may ignite immediately upon being introduced into combustion chamber 10 due to the heat of the compressed intake air. Additionally or alternatively, the main injection may ignite shortly after being injected into combustion chamber 10.

During idling or low load engine operation, the control signal may be altered such that fuel injector 42 may inject fuel into combustion chamber 10 only when piston 18 may be closer to BDC. This may be achieved by, for example, modifying the pilot injection from the first mode, and eliminating the main injection from the first mode. This may be referred to as a second mode of operation for engine 12. The designation of “first” and “second” modes are for explanation purposes only and have no other significance. Moreover, the terms “low load”, “normal load”, and “high load”, may be relative. For example, idle or low load condition for an engine lacking an exhaust gas recirculation (“EGR”) assembly (not shown) may begin at or around 25% load (approximately 600 kPa·BMEP), whereas for an engine having an EGR assembly, idle or low load condition may begin at or around 50% load (approximately 1200 kPa·BMEP). Thus, it should be understood that the meaning of low, normal, and high load will vary by engine, and these designations are not intended to be limiting to the overall disclosed concept.

The control signal may be selectively generated by a controller 48 shown in FIG. 1. Controller 48 may take many forms, including a computer based system, a microprocessor based system including a microprocessor, a microcontroller, or any other suitable control type circuit or system. Controller 48 may also include memory for storage of a control program for operation and control of combustion chamber 10 and related elements. Furthermore, controller 48 may communicate with a set of sensors (not shown) measuring revolutions per minute, engine coolant temperature, oil temperature, piston position, intake manifold pressure, combustion chamber temperature, and/or exhaust temperature. From the sensed information and/or values indicative thereof, controller 48 may determine whether engine 12 is operating at idle, low load, normal, or high load conditions, and may vary the amount of fuel supplied to combustion chamber 10 and/or fuel injection timing accordingly. For example, controller 48 may vary the amount of fuel supplied to combustion chamber 10 by controlling the duration of an injection event, where there is a direct relationship between the length of time fuel injector 42 is open (injecting fuel) and the amount of fuel injected. Also, controller 48 may control the number of fuel injection events, and the timing of each of those combustion events with respect to the combustion cycle. It is also contemplated that controller 48 may vary the amount of fuel and/or fuel injection timing according to instructions from a vehicle operator, vehicle maintenance personnel, and the like. In one embodiment, controller 48 may include an electronic engine control module or fuel injector control module that may be configured to perform the above mentioned functions.

It is also contemplated that controller 48 may divide a single fuel injection event into a series of smaller fuel injection events occurring one after over a brief period of time, such as, for example, over a time period of 3 milliseconds or less. To achieve such an effect, controller 48 may cause fuel injector 42 to shift between opened and closed positions rapidly, similar in manner to a vibratory movement. During the vibratory movement, fuel may be injected when fuel injector 42 moves from closed position to opened position. Utilizing a plurality of small, closely spaced fuel injection events may decrease the likelihood of fuel adhering to the walls of combustion chamber 10, thus allowing for more efficient and complete combustion.

Controller 48 may also operate/actuate intake valve 36 and/or exhaust valve 40. For example, controller 48 may include an intake valve actuator (“IVA”), configured to actuate intake valve 36 such that intake valve 36 may move between an opened position, wherein intake air may flow into and out of combustion chamber 10; a closed position, wherein intake air may not enter or exit from combustion chamber 10; and positions therebetween. During operation of engine 12 in the first mode, IVA may close intake valve 36 at or around the time that piston 18 reaches BDC. In the second mode, the IVA may delay the closing of intake valve 36 until shortly after piston 18 begins to travel towards TDC during a compression stroke. Thus, as piston 18 begins the compression stroke, the intake air within combustion chamber 10 may still exit from combustion chamber 10 through opened intake valve 36. Shortly thereafter, the IVA may close intake valve 36. Because a quantity of intake air has escaped due to the delayed closing of intake valve 36, a smaller volume of intake air may be found in combustion chamber 10 in the second mode than in the first mode. With less air in combustion chamber 10, the combustion of fuel may become more difficult. Thus, in the second mode, piston 18 may be required to travel to a location closer to TDC than would be required in the first mode, in order to create enough heat and pressure within combustion chamber 10 to bring about combustion (ignition of fuel). As such, by using IVA to adjust the amount of intake air in combustion chamber 10, controller 48 may control the timing of combustion with respect to the combustion cycle.

A selective catalytic reduction (“SCR”) system 64 may be in fluid communication with exhaust port 38. SCR system 64 may inject a reduction agent into the exhaust stream of the engine using an injector 67, wherein the reduction agent may include, for example, gaseous ammonia, ammonia in aqueous solution, aqueous urea, or ammonia from an ammonia generator (not shown). Within the exhaust stream, the reduction agent may undergo a hydrolysis process and may decompose into byproducts, including, for example, gaseous ammonia and carbon dioxide. The exhaust stream may be passed over a SCR catalyst 66 that may include aluminum, titanium, or other suitable metal or alloy as a carrier, and platinum, vanadium oxide, iron oxide, molybdenum oxide, or the like as an active member. As the exhaust stream is passed over SCR catalyst 66, the byproducts may react with nitrogen oxides (“NOx”) in the exhaust stream and reduce the NOx to molecular nitrogen. This may reduce or limit the NOx emissions in the exhaust stream. Typically, SCR system 64 may reduce NOx emissions in high temperature exhaust streams with great efficiency, but SCR system 64 may reduce NOx emissions in lower temperature exhaust streams with less efficiency. For example, SCR catalyst 66 may work efficiently when supplied with an exhaust stream having a temperature of approximately 200° C. or greater. However, idle or low load operation of the engine 12 may produce an exhaust stream with a temperature of anywhere between approximately 90 and 200° C., which when supplied to SCR catalyst 66, may not be hot enough to fully initiate the aforementioned reactions.

A diesel particulate filter 68 (“DPF”) may be in fluid communication with SCR system 64 downstream of SCR catalyst 66. DPF 68 may include any type of filter known in the art, such as, for example, a foam cordierite, sintered metal, ceramic, or silicon carbide type filter. DPF 68 may further contain catalyst materials, including, for example, aluminum, platinum, rhodium, barium, cerium, and/or alkali metals, alkaline-earth metals, rare-earth metals, or combinations thereof. Furthermore, at least a portion of DPF 68 may be arranged into a honeycomb, mesh, and/or other suitable configuration that may provide for the filtering of pollutants as the exhaust stream passes through DPF 68.

A method of implementing one aspect of this disclosure is shown in FIG. 4. The method may begin by starting engine 12 (step 50). The next step may include determining one or more parameters associated with engine 12 (step 52). The engine parameters may include the sensed values, values indicative thereof, and/or values calculated based thereon. If one or more of the engine parameters is outside of a predetermined range of values (step 54), then controller 48 may adjust the fuel injection timing such that engine 12 may operate in the second mode (step 56). If the one or more engine parameters is within the predetermined range, then engine 12 may adjust the fuel injection timing such that engine 12 may operate in the first mode (step 58). Until engine 12 is turned off (step 60), controller 48 may continually monitor engine parameters, and based upon this information, may selectively place engine 12 into first and second modes. Once engine 12 is turned off, the process or method may end (step 62).

INDUSTRIAL APPLICABILITY

The disclosed internal combustion engine 12 may have applicability in diesel fueled work machines. Engine 12 may have particular applicability in serving to reduce the amount of undesired emissions in work machine exhaust without significantly increasing overall fuel consumption and/or energy expenditure as a work machine operates under idle or low load conditions.

More demanding emissions standards have necessitated attempts at reducing smoke and NOx byproducts of the combustion process for engines, while maintaining or improving fuel efficiency. Under normal to high engine load conditions, where engine 12 may operate in what may be referred to as a first mode, a relatively high temperature exhaust stream may be produced, which may provide the heat necessary for a selective catalytic reduction (“SCR”) catalyst 66 and/or diesel particulate filter (“DPF”) 68 to operate efficiently to reduce the amount of the smoke and NOx byproducts in the exhaust stream.

Under normal to high load engine conditions, such as, for example, when engine 12 is running above 25% load (approximately 600 kpa·BMEP) if engine 12 lacks an exhaust gas recirculation (“EGR”) assembly, or above 50% load (approximately 1200 kPa·BMEP) if engine 12 has an EGR assembly, engine 12 may operate in the first mode. Operating in the first mode may include injecting a first amount of fuel (a pilot injection) into a combustion chamber 10 using a fuel injector 42, while a piston 18 may be located at or near bottom dead center (“BDC”) position. The pilot injection may mix with intake air already in combustion chamber 10 as piston 18 performs a compression stroke, moving from BDC towards top dead center (“TDC”). The pilot injection/intake air mixture may combust when a sufficient temperature and pressure is achieved. Also, a second amount of fuel (a main injection) may be injected into combustion chamber 10 by fuel injector 42, contemporaneous with or slightly before the combustion temperature of the pilot injection/intake air mixture is achieved. The main injection may ignite upon being injected into the pilot injection/intake air mixture, or immediately thereafter. The heat to ignite the main injection may be supplied by the heat generated by the compression and/or combustion of the pilot injection/intake air mixture. In the first mode, the temperature of the exhaust stream produced by the engine 12 may be at or above 200° C., and thus, the temperature of SCR catalyst 66 may be at or above 200° C., allowing SCR catalyst 66 to efficiently remove NOx from the exhaust stream. Additionally or alternatively, in the first mode of operation, the pilot injection may be omitted, and the main injection may ignite by the heat supplied by compression of the intake air alone, rather than a mixture of intake air and fuel.

Under idle or low load engine conditions, such as, for example, when the engine 12 operates at or below 25% load (approximately 600 kPa·BMEP) when engine 12 lacks an exhaust gas recirculation (“EGR”) assembly, or below 50% load (approximately 1200 kPa·BMEP), when engine 12 has an EGR assembly, engine 12 may operate in a second mode. Under these conditions, the performance of SCR catalyst 66 may sharply decline because the temperature of the exhaust stream produced by engine 12 may fall to a range anywhere between approximately 90 and 200° C. The second mode may be incorporated into the engine cycle of engine 12 to reduce the smoke and NOx byproducts while maintaining or improving fuel efficiency. The second mode may be more accurately referred to as a controlled auto-ignition mode. The second mode combustion process may include substantially simultaneous combustion at a plurality of locations in a combustible mixture, and may eschew the main injection associated with the first mode. This distribution of combustion throughout the mixture may prevent the formation of flame fronts and localized high temperature regions associated with operating engine 12 in the first mode, and thereby may reduce smoke and NOx byproducts in the exhaust stream of engine 12 as it operates under idle or low load conditions.

The operation of engine 12 in the second mode may include the steps of providing air into combustion chamber 10, injecting fuel into combustion chamber 10 using fuel injector 42 to allow the air and fuel to form a homogeneous mixture, and compressing the homogeneous mixture in combustion chamber 10 until it auto-ignites. Auto-ignition may occur when the heat of the compressed gas is sufficient to ignite the fuel. The mixture of air and fuel may be homogeneous when it has uniform composition, appearance, and properties throughout the mixture. The air and fuel may be heterogeneous where the fuel and air are substantially distinct and have not entirely mixed.

In some embodiments, the injecting step may be initiated during a range of piston positions spanning from approximately 90 to 40 degrees before TDC. The exact timing of the fuel injection may be controlled electronically by a controller 48, and optimal timing may depend on both the engine design as well as its speed and load. It is also contemplated that other gases may be provided to combustion chamber 10, such as, for example, exhaust gases supplied exhaust gas recirculation (“EGR”) system (not shown), which may increase the temperature of the compressed homogeneous mixture to assist in bringing about auto-ignition. It is further contemplated that controller 48 may also affect the timing of auto-ignition by controlling and/or delaying the opening and closing of an intake valve 36 that selectively brings combustion chamber 10 into and out of fluid communication with an air intake manifold (not shown) of engine 12.

FIG. 3, which is representative of the operation of engine 12 in the second mode during idle or low load conditions, illustrates the compression stroke of piston 18 at a piston position of approximately 50 degrees before TDC. At this point in the combustion cycle, intake air may have entered the combustion chamber 10 and may be compressed and mixed with fuel injected from nozzle 44. As noted above, other gases may exist in combustion chamber 10, for example exhaust gases may be present by way of the EGR system (not shown). The injected fuel, for example diesel fuel, may form fuel plumes 70 within combustion chamber 10. As piston 18 progresses toward TDC from 50 degrees before TDC, the fuel and air may have sufficient time to homogeneously mix together. The homogeneous mixture may be compressed and may eventually auto-ignite when the pressure in combustion chamber 10 exceeds a threshold auto-ignition pressure of the homogeneous mixture. As understood in this disclosure, initiation of fuel injector 42 may correspond to controller 48 sending an electrical control signal energizing fuel injector 42 for fuel injection, and or operating one or more valves (not shown) to accomplish the same. By controlling fuel injector timing, controller 48 may selectively place engine 12 into first and second modes depending on engine load conditions.

The ability to switch engine operation from the first mode during normal to high load conditions, into the second mode during idle or low load conditions, may be desirable for many reasons. The second mode combustion process may provide reduced smoke, reduced NOx, and a reduction in unburned hydrocarbons in the exhaust stream of engine 12 by reducing the formation of detrimental high temperature regions within combustion chamber 10, thus resulting in improved emissions and better fuel economy during idle and low load conditions. Furthermore, due to the reduction of the amount of pollutants in the exhaust stream, frequent blocking or clogging of SCR catalyst and/or DPF by trapped pollutants may be avoided, and so regeneration may be carried out less often or not at all while engine 12 operates in the second mode. Since regeneration may typically require additional fuel and/or energy, avoiding regeneration may reduce fuel and/or energy costs associated with operating engine 12.

It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed system and method without departing from the scope of the disclosure. Additionally, other embodiments of the disclosed system and method will be apparent to those skilled in the art from consideration of the specification. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents. 

1. A method of operating an internal combustion engine, comprising: selectively operating the engine in a first mode of operation, wherein fuel is injected into compressed fluid to create a substantially heterogeneous mixture of fuel and compressed fluid, and fuel is ignited by the heat of the compressed fluid; selectively operating the engine in a second mode of operation, wherein fuel is injected into gas to create a substantially homogeneous mixture of gas and fuel, the substantially homogeneous mixture is compressed, and fuel is ignited by the heat of the compressed gas; and determining whether to operate the engine in the first or second mode according to engine conditions.
 2. The method of claim 1, further including providing the engine with at least one piston, wherein fuel is injected closer to a top dead center position of the piston in the first mode than in the second mode.
 3. The method of claim 1, further including decreasing the volume of gas in the second mode to delay the ignition of fuel.
 4. The method of claim 1, further including providing the engine with an SCR catalyst, wherein the engine is operated in the second mode when the temperature of the SCR catalyst is below a threshold level.
 5. The method of claim 4, wherein the threshold level is approximately 200° C.
 6. The method of claim 1, wherein the fluid is a mixture of gas and fuel.
 7. The method of claim 1, further including operating the engine in the second mode when the engine is at idle.
 8. The method of claim 1, wherein in the second mode of operation, a plurality of fuel injections occur prior to ignition.
 9. An internal combustion engine, comprising: a combustion chamber including a piston and a cylinder; and a fuel injection controller that selectively operates the engine in first and second modes of operation, and selects the mode according to engine conditions; wherein in the first mode, fuel is injected into compressed fluid to create a substantially heterogeneous mixture of fuel and compressed fluid, and fuel is ignited by the heat of the compressed fluid; and wherein in the second mode, fuel is injected into gas to create a substantially homogeneous mixture of gas and fuel, the substantially homogeneous mixture is compressed, and fuel is ignited by the heat of the compressed gas.
 10. The engine of claim 9, wherein fuel is injected closer to a top dead center position of the piston in the first mode than in the second mode.
 11. The engine of claim 9, wherein the fuel injection controller operates the engine in the second mode when the engine is at idle.
 12. The engine of claim 9, further including an SCR catalyst, wherein the engine is operated in the second mode when the temperature of the SCR catalyst is below a threshold level.
 13. The engine of claim 12, wherein the threshold level is approximately 200° C.
 14. The engine of claim 9, wherein the fluid is a mixture of gas and fuel.
 15. A work machine having an internal combustion engine, comprising: a combustion chamber including a piston and a cylinder; and a fuel injection controller that selectively operates the engine in first and second modes of operation, and selects the mode according to engine conditions; wherein in the first mode, fuel is injected into compressed fluid to create a substantially heterogeneous mixture of fuel and compressed fluid, and fuel is ignited by the heat of the compressed fluid; and wherein in the second mode, fuel is injected into gas to create a substantially homogeneous mixture of gas and fuel, the substantially homogeneous mixture is compressed, and fuel is ignited by the heat of the compressed gas.
 16. The work machine of claim 15, wherein fuel is injected closer to a top dead center position of the piston in the first mode than in the second mode.
 17. The work machine of claim 15, wherein the fuel injection controller operates the engine in the second mode when the engine is at idle.
 18. The work machine of claim 15, further including an SCR catalyst, wherein the engine is operated in the second mode when the temperature of the SCR catalyst is below a threshold level.
 19. The work machine of claim 17, wherein the threshold level is approximately 200° C.
 20. The work machine of claim 15, wherein the fluid is a mixture of gas and fuel. 